This invention relates to genetically modified plants, and in particular to genetically modified maize and wheat. The genetically modified plants have an altered starch synthesizing ability following the introduction, by recombinant DNA techniques, of one or more gene sequences coding for enzymes in the starch or glycogen biosynthetic pathway into the plant.
Starch is a complex polymer of glucosyl residues. It is the major form in which carbohydrate is stored in the tissues and cells of most species of higher plants. It is accumulated in the leaves of plants during the day as a result of photosynthesis and is used to supply the needs of the plant for energy and biosynthesis during the night. Starch is also accumulated in non-photosynthetic cells, especially those involved in reproduction such as in seeds, fruits and tubers. Therefore, starch is of great importance to the productivity of the plant and its survival.
Starch is also highly significant to man. Firstly, it forms a major component of animal diets, supplying man and his domestic animals with a large portion of their carbohydrate intake. Secondly, the type of starch in a plant affects the quality of the processed plant product. Thirdly, starch is used industrially in the production of paper, textiles, plastics and adhesives, as well as providing the raw material for some bioreactors. Starch from different species have preferred uses. On a world scale, starch producing crops are agriculturally and economically by far the most important, and these crops include wheat, maize, rice and potatoes. The type of starch will affect the quality of a processed product and the profitability of the processed crop. In addition, the quantity and quality of starch present in the harvested organ of a plant will affect the gross yield and the processing efficiency.
In plants, i e., vascular plants, the starch consists of linear chain and branched chain glucans known as amylose and amylopectin respectively. Starch with various amounts of amylose and amylopectin are found in different plants. Typically, plant starch contains 10-25% amylose, the remainder being amylopectin, the branched chain glucan. Amylopectin contains short chains and long chains, the short chains ranging from 5-30 glucose units and the long chains ranging from 30-100 glucose units, or more. It is thought that the ratio of amylose to amylopectin and the distribution of short to long chains in the amylopectin fraction affect the physical properties of starch, e.g., thermal stabilization, retrogradation and viscosity. These properties also affect the utility of starch, as mentioned above. Starches from different plants have different properties, which also affects their suitability for processing under certain conditions and for certain uses. It can be seen, therefore, that modifying the starch generated in a plant can have particular utility in the downstream processing or the yield of the starch in the plant storage organ.
For example, waxy corn starch lacks amylose and this starch has unique properties. Also, most mutations in the waxy locus of maize, which encodes starch granule bound synthase I (GBSSI), result in plants which produce much reduced amylose. When no functioning GBSSI is synthesized in the homozygous waxy mutant, it also lacks amylose (Echt and Schwartz, 1981).
The genetic modifications of the present invention produce altered starch composition and properties, which properties are ideally beneficial in terms of starch processing.
This invention seeks to transform cereal crops and specifically wheat and maize with an enzyme involved in the synthesis of microbial glycogen, namely glycogen synthase (E.C.2.4.1.21).
This invention also seeks to modify properties of the starch in these transformed plants which are particularly useful and/or advantageous in the downstream processing of starch or the plant itself.
The present invention provides transgenic wheat or maize plants, said plants having therein a chimeric gene comprising a promoter, operably associated with a coding sequence for glycogen synthase, and a terminator. The genetically modified plants have an altered starch synthesizing ability following the introduction, by recombinant DNA techniques, of a coding sequence for glycogen synthase.
In another embodiment, more than one gene sequences coding for enzymes in the starch or glycogen biosynthetic pathway may be introduced into the plant in addition to the coding enzyme for glycogen synthase.
The present invention provides a method of altering the starch in maize or wheat plants, the method comprising the steps of stably introducing into the plant genome a nucleic acid sequence encoding glycogen synthase under the direction of a suitable promoter and a suitable terminator, and regenerating a plant having an altered genome.
The present invention also provides novel forms of starch which can be obtained from said transgenic cereal crop plants, e.g., transformed wheat or maize, and which is characterized by an altered chain length and/or modified processing properties as compared to starch prepared from a non-transgenic cereal crop plant. A composition comprising the altered starch obtained from the transgenic plants is contemplated. Preferably, the altered starch is purified.
Plant cells containing a chimeric gene comprising a nucleic acid sequence encoding glycogen synthase are also an aspect of this invention, as are other plant parts, such as for example, seed of the transformed plant containing a chimeric gene according to the invention.